I’m kinda iffy on this

On one hand, 2004 was the year when emo got popular with the likes of My Chemical Romance and Simple Plan. On the other hand, Ushers Confessions sounds almost similar to his 2001 album 8701.

2001 for sure. I remember both years vividly. 2001 and 2004 had similar R&B/hip hop. 2004 was a transitional year from early 2000’s into mid 2000’s. But all in all I say music in 2001 was very similar to what was being produced in 2004.

Hip hop:

Project Pat- Chicken head (2001)

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Destiny’s Child- Soldier (2004)

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R&B
Usher- U don’t have to call (2001)

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Mario Winans- I don’t wanna know (2004)

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Pop rock

Sum 41- Into deep (2001)

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Bowling for soup- 1985 (2004)

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2004 was overall in the same era as 2001 with glam rap and slow jamz style R&B. Late 2005 was when the snap/ringtone, emo rock, and "new new jack swing" became popular which was prevalent in 2007.

2007.
This was the year emo replaced nu metal as the definitive angsty rock genre. Post-grunge saw a shift to edgy alt-metal type bands like Three Days Grace, Seether, and Breaking Benjamin, bands also popular in 2007. Even non-emo pop punk was becoming more serious with albums like American Idiot and Under My Skin. This was also the year Eminem went downhill with Encore, and Kanye West emerged on the scene.
As far as what is similar to 2001, ringtone rap, at its peak in 2007, did not exist in 2001 or 2004. OutKast was huge in 2004 much like in 2001, but by 2007, they were irrelevant. Britney Spears didn’t have her meltdown yet.
Otherwise, I generally find more similarity to 2007, at least when it comes to rock and rap. Rock especially I actually tend to group 2004-2007 as one era.